Cordierite bodies having honeycomb structures are especially suited for but not limited to use as substrates for catalysts for converting automotive exhaust, for example, or as diesel particulate filters or as regenerator cores. Use of cordierite is favorable in these applications because of its good thermal shock resistance. The thermal shock resistance is inversely proportional to the coefficient of thermal expansion (CTE). That is, honeycombs with low thermal expansion have good thermal shock resistance and can survive the wide temperature fluctuations that are encountered in the application.
Although the mineral cordierite has an intrinsically low CTE (about 17.times.10.sup.-7 .degree.C..sup.-1 (25-800.degree. C.)), cordierite ceramics formed by the reaction of certain simple or complex natural or synthetic raw materials (e.g. kaolin+talc+alumina; magnesia+alumina+silica; spinel+silica) can exhibit CTE's that are much lower. The attainment of these low expansions is dependent on three microstructural features related to the nucleation and growth of the cordierite: microcracking, crystal orientation, and residual phases.
Microcracking is dependent on the anisotropy in the thermal expansion of cordierite along its crystallographic axes. Thermal stresses generated during cooling after firing result in microcrack formation. During reheating, some of the thermal expansion of the ceramic body is accommodated by the re-closing of the microcracks, yielding a reduction in the bulk CTE of the ceramic. The presence of microcracking in a ceramic body is manifested by hysteresis in the thermal expansion curve for that body.
The development of a non-random orientation of the cordierite crystals during sintering also influences thermal expansion. The extrusion of cellular bodies of cordierite-forming batches imparts an alignment or foliation of the tabular and plate-like raw materials, which in turn results in the growth of cordierite crystals in which the negative-expansion c-axes of the cordierite crystals tend to lie within the plane of the honeycomb walls. This microstructural feature further contributes to a reduced CTE in both the axial and radial dimensions of the honeycomb. The extent to which the cordierite crystals are oriented with their c axes in the plane of the cell walls of the honeycomb is measured by x-ray diffractometry (XRD) of the as-fired surfaces of the walls. Specifically, the XRD intensities of the (110) and (002) reflections from the cordierite crystals (based upon hexagonal indexing) are determined for the as-fired surface of the cell wall. The intensity of the (110) reflection, I(110). is proportional to the fraction of crystals lying with their c axes in the plane of the wall, while the intensity of the (002) reflection, I(002), is proportional to the fraction of crystals growing orthogonal to the cell wall. An "I-ratio" is defined by the following relation: ##EQU1##
The I ratio ranges from 0.0 for a body in which all of the cordierite crystals are oriented with their c axes perpendicular to the cell wall, to 1.00 for a body in which all of the crystals lie with their c axes within the plane of the wall. It has been found experimentally that a body in which the cordierite crystals are randomly oriented exhibits an I-ratio equal to approximately 0.655.
Finally, to achieve a low-CTE cordierite body, it is necessary that the reaction of the cordierite-forming raw materials proceed essentially to completion so that there exists a minimum of residual, high-expansion phases such as glass, cristobalite, mullite, alumina, spinel, and sapphirine in the body after sintering.
The formation of low thermal expansion cordierite bodies is dependent therefore on the nucleation of the cordierite raw materials and on its subsequent growth. An interplay exists between firing schedule and the nature of the raw materials such that a batch that yields a low-CTE ceramic when fired on a long schedule may result in a high expansion on a shorter schedule and vice versa. Specifically, shortening of the schedule may result in incomplete reaction of some batches, resulting in a CTE increase due to the presence of residual, high expansion phases. Alternately, faster firing rates and shorter schedules could decrease the expansion of other batches (provided that the reaction to form cordierite is nearly complete) by increasing the amount of microcracking.
At present cordierite-forming cellular ceramics are fired at heating rates of about 10 to 200.degree. C./hr. through specific segments, with average heating rates of approximately 30 to 70.degree./hr. from 25.degree. C. to maximum temperature. Soak times at maximum temperature range from about 6 to 12 hours, and total firing cycles are greater than about 25 hours in duration.
A method for producing low thermal expansion cordierite bodies in which the total firing time is substantially less than 20 hours, and preferably less than 5 hours would have several advantages. Among the advantages would be more efficient use of equipment, less energy consumption, and greater productivity.